Cases for a storage device such as a storage battery and a capacitor are roughly classified into two types, i.e., mainly, a type where a cylindrical or rectangular can is formed using a metal plate by press working, clamping, laser welding, etc., and a pouch type where a resin film having a metal foil as a gas barrier layer is used and heat-sealed to form a case (in this case, the case is soft and therefore, is also referred to as a bag body).
The battery of a pouch type is used in a state that a storage device part is cut off from the outside world by wrapping the storage device by metal foils each laminated with a heat-sealing resin (laminated metal foils) and heat-sealing the heat-sealing resins thereof to each other. This is because leakage of an electrolytic solution of a battery to the outside or mingling of water vapor into a battery from the environment is fatal to the battery life.
However, in the case of a conventional battery cell where laminated metal foils are joined only by heat sealing, the heat-sealed portion may serve as a leakage path for an electrolytic solution within the battery or an intrusion path through which water vapor or the like enters into the inside of the battery from the external environment, and the path length of the heat seal part contributes to determination of the life of the battery cell. Therefore, for extending the life of the battery cell, it may be effective to elongate the path length of the heat seal part, but on the other hand, when the path length of the heat seal part is elongated, a useless space is increased and the cell capacity per space is decreased. Accordingly, in the battery cell using a laminate pack joined by heat sealing, a trade-off relationship exists between the cell capacity per unit space and the battery life.
Incidentally, a laminated aluminum foil has been heretofore employed as the laminated metal foil used in the pouch-type battery case. This is related to the characteristics of aluminum, i.e., ease of obtaining a thin metal foil, and at the same time, related to the fact that the pouch-type case has been developed from a resin pouch/bag body for food packaging. More specifically, in a food packaging pouch/bag, aluminum has been deposited as a barrier layer so as to impart a gas barrier property and thereby extend the life of food. In the case of applying this pouch/bag to a lightweight battery container capable of being easily joined by heat sealing, since the gas barrier property required of, among others, a lithium ion battery or the like using a non-aqueous electrolyte is by far higher than that for food, the reliability of the gas barrier layer must be enhanced. To meet this requirement, the thickness of the gas barrier layer of aluminum has been increased, and as a result, the application of an aluminum deposited film has been shifted to the application of an aluminum foil.
For example, Patent Document 1 (Kokai (Japanese Unexamined Patent Publication) No. 2010-086744) discloses “a packaging material for an electrochemical cell, fabricated by sequentially stacking at least a base material layer, a metal foil layer with a chemical conversion-treated surface, an acid-modified polyolefin layer and a thermal adhesive resin layer”, as an outer package for hermetically housing an electrochemical cell body such as a lithium ion battery body, a capacitor and an electric double-layer capacitor or as a packaging medium for a battery housing. In this technique, the “base material layer” is a resin film, and it is understood only from this expression that the role of the metal foil layer is incidental. Actually, in the description, it is said that “the metal foil layer 12 is a layer for preventing intrusion of water vapor into the inside of the lithium battery from the outside”.
Patent Document 2 (Kokai No. 2000-340187) describes, as a packaging material for a polymer battery, “ . . . a packaging material for a polymer battery, consisting of an outermost layer/a barrier layer/an intermediate layer/an innermost layer”, and clearly specifies that the metal foil layer (aluminum foil layer) is a barrier layer.
Patent Document 3 (Kokai No. 2000-153577) also states that other than an aluminum foil disclosed as a working example of the metal foil of a heat-sealing laminate, a stainless steel foil can be used.
However, the junction part heat-sealed by the resin laminated is disadvantageous in that since the heat seal part is not formed of a metal, the junction part is composed of only a resin and does not have a gas barrier property comparable to that of other portions having a metal layer as a barrier layer or of a junction part composed of a metal, such as welded metal can, and a sufficient gas barrier property cannot be exerted particularly in a battery where intrusion of water has a fatal effect on the life and therefore, a high gas barrier property is required.
In this connection, a method of welding a laminated metal foil and thereby applying sealing with a resin and sealing with a metal is disclosed in Patent Document 4 (Kokai No. 2000-223090) and Patent Document 5 (Kokai No. 2008-021634).
The method of Patent Document 4 is a method where in a laminate cell that is folded in half and sealed at its circumference two sides are welded to enhance the gas barrier property.
However, in the case of welding a laminated metal by melting the metal, the welding by an ultrasonic wave or conduction heating requires metals to come into direct contact with each other, and welding by a heat source using a laser or a discharge arc has a problem that the molten metal is blown off due to evaporation of the resin and a sound weld bead can be hardly formed.
Therefore, Patent Document 4 sets forth, in paragraph [0007] of [Means to Solve the Problems], a description of “on the outer side, the heat-fusible resin film layer is removed to expose the metal foil surface and metal foils are laid one on another and welded”, and a step of previously removing the resin in the portion to be welded, like the W part in FIG. 4(c) of Patent Document 4, is indispensable. Furthermore, a method for removing the heat-fusible resin film as above and a method for pressing and superposing the portions to be welded are additionally required.
In Patent Document 5, also as shown in FIGS. 2 and 3, a step of providing a taper on the end face and previously removing the resin inside the portion to be welded is necessary.
In addition, the normal welding method is generally a welding method where as in FIG. 4(e) of Patent Document 4, a weld metal is formed between metal foils put into contact, but this method has an additional problem that occurrence of a welding defect can be hardly detected and it is difficult to guarantee the soundness of the weld part so as to ensure the barrier property.